Network Working Group J.C. Klensin
Internet-Draft A. Sullivan
Intended status: Informational Dyn
Expires: September 25, 2014 P. Faltstrom
Netnod
March 26, 2014
An IANA Registry for Protocol Uses of Data with the DNS TXT RRTYPEdraft-klensin-iana-txt-rr-registry-02
Abstract
Some protocols use the RDATA field of the DNS TXT RRTYPE for holding
data to be parsed, rather than for unstructured free text. This
document specifies the creation of an IANA registry for protocol-
specific structured data to minimize the risk of conflicting or
inconsistent uses of that RRTYPE and data field.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 25, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
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extensive discussions about whether that type of use of the TXT
RRTYPE is appropriate or desirable; design choices about DNS
extensions and some of their consequences are discussed in RFC 5507
[RFC5507]. However, independent of how one feels about those issues,
the reality is that the DATA fields of TXT RRs are in use for
protocol-specific information that is interpreted by the protocols
themselves. Those uses are not going to disappear. It is even
possible that tradeoffs between established uses, conversion costs,
and related issue might justify standardization of the practice,
however problematic and/or distasteful that might be in principle.
Having structured information that is protocol-specific without a
registry increases the risk of different parties using the same
identifying information for different purposes, thereby creating
security and operational risks. This document specifies the relevant
registry.
It might be argued that a registry is inappropriate, because it is in
effect a subtyping of the TXT RRTYPE. While the position has merit,
without a registry and with continued uses of TXT to support pieces
of protocol, it is only a matter of time before overlapping or
confusable uses turn into an attack. While such an outcome might be
accidental, it would still be bad. If there were a registry, then
one might dream of zone-checking tools that would warn about
apparently-structured information that didn't reflect any of the
registered entries.
It is important to stress that this registry is intrinsically about
what is being done and not about what risks exist and whether
particular measures or considerations might mitigate those risks.
This document specifies creation of that registry and the means by
while it is populated.
2. Registry Contents2.1. General Usage Information
Each registry entry consists of a reference to the protocol that
identifies specific information in the TXT Resource Record's RDATA
field and that indicates what information is used to make that
identification. For example, if the "foo" protocol were described in
RFC 9999 and used the presence of the string "foo=" at the beginning
of the first character string in the TXT Resource Record RDATA to
identify information that applied to it, the registry would identify
the protocol ("foo"), the submitter, the reference that described it
("RFC 9999") and the association-determining string ("foo="). In
addition, the registry will identify any constraints on or special
properties of the RDATA, such as whether it is restricted to ASCII,
expected to be in UTF-8 characters, may contain more than one string,
or represents some other type of information. The registry also
allows for comment information. That information might include
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information about prefixes or suffixes for the DNS owner name that
are used with the particular protocol at issue.
For tracking purposes, each entry also contains date created and date
last modified information.
2.2. Character Sets, Codes, and RestrictionsSection 3.3.14 of the DNS Specification [RFC1035] specifies only that
the RRDATA for TXT consists of "One or more <character-string>s".
Section 3.2 specifies that a <character-string> "is a single length
octet followed by that number of characters. <character-string> is
treated as binary information, and can be up to 256 characters in
length (including the length octet).". If more that one string is
allowed for the particular TXT record, the registration record should
indicate that fact and how the multiple strings are interpreted.
Similarly, if the character strings are not restricted to ASCII
characters, the registration record should discuss how they are to be
interpreted, what constraints are applied, and, if the strings are
likely to be compared to others, what collation sequences or other
rules apply.
3. Adding New Registry Entries
As discussed when the IETF concluded that there should be fewer
barriers to the creation and use of new RRTYPES [RFC6895] best
practices today generally call for creating new, protocol- or
application-specific types rather than overloading information onto
the TXT RRTYPE. Consequently, this registry is expected to reflect
deployed existing practices rather than new uses for TXT. Its use to
make the latter more acceptable would contradict the intent of both
this specification and the registry itself.
Consistent with that principle, the procedure for accepting new
entries into the registry will be review by a Designated Expert
[RFC5226] as modified below.
The Expert is expected to determine that the particular use of TXT is
in established use and, ideally, is documented with a stable
specification as defined by RFC 5226 and the RFC Editor. The
existence of a standards track RFC or equivalent specification is
always sufficient to meet those conditions. In less common cases,
the Expert should consider the explanation above and apply good
judgment, favoring adding entries to the registry in cases of doubt.
Cases that cannot be resolved adequately by discussion between the
applicant and the Expert may be referred to the IESG.
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Registries may be updated using the same mechanisms used to create
new ones. The expert reviewer should attempt to ensure that updates
are limited to corrections or consistent expansions of earlier
information and that the party proposing the update is at least as
authoritative about the original protocol as the party who submitted
the original registration request.
5. Registration Template
A registration request should supply the following information,
ideally in this form:
1. Name and contact information for the submitter.
2. Protocol identification and, where feasible, documentation
reference.
3. Distinguishing characteristics of the TXT RDATA content that
permit identifying information for this protocol.
4. Character set information and limitations on the RDATA field or
an indication that the RDATA are not in character form and
whether more than character string is permitted. If more than
one character string is permitted, this section of the template
should note that and discuss how multiple strings are to be
interpreted. This field may be a reference to a stable external
document.
5. Any special considerations for multiple TXT records with the same
owner name.
6. If feasible, an explanation of why the TXT RRTYPE is preferred
for this particular use over a dedicated, purpose-specific
RRTYPE.
7. Other special constraint or identifying information. For
example, if the protocol being registered requires special
prefixes or suffixes for the owner name, a discussion of that
requirement.
6. Internationalization Considerations
As discussed above, the DNS protocol does not restrict TXT RDATA
fields to ASCII characters. If appropriate, other character
repertoires and encodings, or even octets interpreted as non-
character binary information, may be used. For reasons discussed in
detail elsewhere, if non-ASCII character data is needed, Unicode
encoded in UTF-8 is strongly preferred to other encoding forms or
script-specific encodings. Because the use of non-ASCII characters
often raises multiple issues, specifically including string
comparison choices, registration information should ideally include a
discussion of relevant issues (or why there is no issue).
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For readers needing further information on this subject, different
aspects of the string comparison problem for non-ASCII text appear in
RFC 2130 [RFC2130], RFC 4790 [RFC4790], RFC 5894 [RFC5894], RFC 6055
[RFC6055], and RFC 6885 [RFC6885].
7. IANA Considerations
This memo specifies the creation of a new registry in the Domain Name
System (DNS) Parameters group [IANA-DNS-Parameters]. The details for
the contents and registration requirements for that registry appear
in Section 2 and Section 3 above.
The registry should have explicit text referencing this document.
The text should be similar to the following:
"This registry exists to decrease the risk for overlapping use of
the TXT Resource Record Type for structured data instead of having
the RDATA for that type contain only descriptive text without
specified semantics (as described in RFC 1035. See RFC 5507 and
the Security Considerations section of RFC NNNN for more
information."
While it is common practice for registry-creating documents to
specify the initial content of the registry, this one deliberately
does not do so in order to allow the actual users of the relevant
types to identify them and provide explanations for their use.
[[Note in draft: The authors disagree as to whether this document
should include all known uses of TXT RRTYPE DATA (specifically
including all of those discovered in the surveys conducted in
conjunction with the SPFbis work) or should create an empty
registry and await addition of entries as described in this
specification. The advantage of the former is that the registry
will contain a useful list of applications of TXT much more
quickly. Via the expert review process, the latter would
presumably yield better descriptions and information about
responsible parties for many entries. A possible middle ground
would be to try to identify all uses that are explicitly
identified in IETF standards track documents and include them
initially, leaving other uses to the registration process.]]
8. Security Considerations
The creation and use of this registry should help to minimize the
risks of different protocols inadvertently using data embedded in TXT
Resource Records in incompatible ways. Consequently, it should have
a positive effect on security. Because this document and the
registry do not address the question of what protocols, if any,
should use TXT RDATA in this way, questions associated with the usage
and structure of particular protocols lie outside its scope.
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There is a general (although by no means unanimous) view among DNS
experts that overloading RRTYPEs, especially the TXT type, is a bad
practice that could lead, not only to the sort of conflicts that this
registry might help prevent, but to requirements for multiple queries
and even an increased risk from amplification attacks. While caution
is always appropriate when documenting a risky practice lest the
documentation be taken as endorsement, the arguments for providing
documentation for deployed protocol uses of TXT RRs seems very
similar to the historical arguments for documenting security risks:
being secretive about them won't prevent either their being exploited
or prevent new ones from being invented.
9. Acknowledgements
The requirement for this registry became obvious as a result of the
discussion of handing records for SPF in the DNS. The comments of
the participants on all sides of that discussion are gratefully
acknowledged.
Specific comments and text for this draft were provided by Eliot Lear
and Subramanian Moonesamy. Dick Francis pointed out the need for a
discussion of non-ASCII characters.
[[Several useful comments about the general approach taken in the
document were received in private notes whose authors may or may not
want to be linied to the draft. The originators of such comments
should contact the first-listed author if they would like to be
listed explicitly.]]
10. References10.1. Normative References
[IANA-DNS-Parameters]
IANA, "Domain Name System (DNS) Parameters", 2013, <http:/
/www.iana.org/assignments/dns-parameters/dns-
parameters.xhtml>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
10.2. Informative References
[RFC2130] Weider, C., Preston, C., Simonsen, K., Alvestrand, H.T.,
Atkinson, R., Crispin, M. and P. Svanberg, "The Report of
the IAB Character Set Workshop held 29 February - 1 March,
1996", RFC 2130, April 1997.
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[RFC4790] Newman, C., Duerst, M. and A. Gulbrandsen, "Internet
Application Protocol Collation Registry", RFC 4790, March
2007.
[RFC5507] IAB, Faltstrom, P., Austein, R. and P. Koch, "Design
Choices When Expanding the DNS", RFC 5507, April 2009.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[RFC6055] Thaler, D., Klensin, J. and S. Cheshire, "IAB Thoughts on
Encodings for Internationalized Domain Names", RFC 6055,
February 2011.
[RFC6885] Blanchet, M. and A. Sullivan, "Stringprep Revision and
Problem Statement for the Preparation and Comparison of
Internationalized Strings (PRECIS)", RFC 6885, March 2013.
[RFC6895] Eastlake, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, April 2013.
Appendix A. DNS Parameter and Registry Loose Ends
[[RFC Editor: please remove this appendix before publication.]]
The discussions surrounding this draft suggest that one or two
additional DNS registries may be needed and that the current IANA
structure for DNS-related information may not be optimal. In
particular, there is no registry for two of the five extension
mechanisms described in RFC 5507 as adding prefix or suffixes to
owner names. In that context, the registry proposed in this
specification can be seen as essentially a subtype registry for the
TXT RRTYPE altnough it is deliberately designed to include TXT-
related prefixes and suffix approaches as well. It would, however,
probably be useful for someone to investigate and, if appropriate,
specify additional subtype, prefix, and suffix registries as
appropriate.
As part of that effort, it may be useful to advise IANA as to whether
some or all of the registries at
https://www.iana.org/assignments/s-naptr-parameters/s-naptr-parameters.xhtml,
https://www.iana.org/assignments/sip-table/sip-table.xhtml,
https://www.iana.org/assignments/enum-services/enum-services.xhtml,
https://www.iana.org/assignments/im-srv-labels/im-srv-labels.xhtml,
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